Preliminary Water Testing - Post Acute Mass Fish Mortality in Monee Reservoir

Abstract

On August 13th, 2025, an unusual amount of fish were seen deceased, floating along the shorelines inside the Monee Reservoir. The fish appeared to be a possible mix of largemouth bass, black crappies & bluegills. All seemed to have died within the last 48 hours, most of which appeared to only be within 12-36 hours of the decomposition stages. None showed signs of trauma or disease related death. The weekend of the 16th after the fish had been removed or sank out of view. The same mass mortality incident was noted occurring again the morning of the 19th. Only some were photographed & only a small area along the shore was observed (30-40%) in one of several ponds. Approximately 8-10 were seen the first day & 4 on the second day. 

The following is a simple screening & preliminary test of water quality to begin narrowing down potential causes, whether natural or man made. 

The EnvioTEC laboratory is undergoing a multi-year long renovation & is currently not equipped to test for many chemical based pollutants or detect contaminants at low-trace levels. This is only a preliminary screening. Further testing would need to be done to confirm results, this is simply a report of our findings & observations.

Methods

Sample #1 - 08/24/25 - 8:15AM

Sample #1 was taken approximately 4-6 inches beneath the surface about 2-3ft out from the shoreline, in the main reservoir area with an active aerator in the center approximately 300ft away. Sample was analyzed unfiltered & filtered using 0.45um syringe filters

TDS, EC, pH, & temperature were measured both directly in reservoir & in sample. 

Sample #2 - 08/27/25 10:30AM

Sample set #2 was taken approximately 6-8in beneath the surface about 4-5ft out from the shoreline. TDS, EC, pH, ORP, salinity & temperature were measured both directly in reservoir & in sample.

Sample #3 08/27/25 - 6:30PM

Sample #3 was taken approximately 6-8in beneath the surface about 4-5ft from the shoreline. Sample was analyzed unfiltered & filtered using 0.45um syringe filters. TDS, EC, pH, ORP, salinity & temperature were measured both directly in reservoir & in sample.

All tests for each sample were performed x2 for validation whenever possible. Electronic meters were calibrated before use and probes washed after every use. 

Sample Sites & Confirmed Dead Fish Sightings

Results

Site A: 08/24 - 8:15AM

Direct in Reservoir

-Temp: 19.8°C, EC: 1074µS/cm, TDS: 537ppm, pH 7-8

Sample (Unfiltered)

-DO: 3.5ppm/3.5ppm

-Temp: 19.9°C, EC: 916µS/cm, TDS: 458ppm, pH 7.8-8.2

-Hardness: 50ppm/75ppm, sodium chloride: 500ppm+/500ppm+, total alkalinity: 100ppm/40ppm, carbonate: 120ppm/80ppm, total chlorine: 0.5-1ppm/0.5-1ppm*

-No free chlorine (bromine) iron, copper, mercury, lead, zinc, fluoride, chlorine, manganese, QAC/QUAT, hydrogen sulfide, nitrates/nitrites, sulfates or ammonia detected or at detectable levels with current equipment.

0.45um filtered

-pH 7.2-7.6

-Hardness: 50ppm, sodium chloride: 500ppm+, total alkalinity: 40ppm, carbonate: 80ppm, total chlorine: 0.5-1ppm

-No free chlorine (bromine) iron, copper, mercury, lead, zinc, fluoride, chlorine, manganese, QAC/QUAT, hydrogen sulfide, nitrates/nitrites, sulfates or ammonia detected or at detectable levels with current equipment.

Site B: 08/27 - 10:30AM

Direct in Reservoir

-Temp: 23.7°C/22.5°C, EC: 854µS/cm / 797µS/cm, Salinity: 0.03%, ORP: 154mV TDS: 427ppm/390ppm, pH 9.2

Sample (Unfiltered)

-DO: >15ppm/>15ppm

-Temp: 26.9°C/25.7°C, EC: 822µS/cm / 777µS/cm, Salinity: 0.03%, TDS: 458ppm, pH 9.1/9.0

-Hardness: 75ppm/75ppm, sodium chloride: 500ppm+/500ppm+, total alkalinity: 100ppm/40ppm, carbonate: 200ppm/200ppm, total chlorine: Error/0.5-1ppm*

-No free chlorine (bromine) iron, copper, mercury, lead, zinc, fluoride, chlorine, manganese, QAC/QUAT, hydrogen sulfide, nitrates/nitrites, sulfates or ammonia detected or at detectable levels with current equipment.

Site C: 08/27 - 6:30PM

Direct in Reservoir

-Temp: 24.2°C EC: 815µS/cm Salinity: 0.04% ORP: 126mV TDS: 406ppm pH: 9.0

Sample (Unfiltered)

-DO: >15ppm/>15ppm

-Temp: 23.9°C/24.1°C, EC: 819µS/cm / 902µS/cm, Salinity: 0.04%, ORP: 164mV TDS: 402ppm/451ppm, pH 9.0/9.0/9.0

-Hardness: 50ppm, sodium chloride: 500ppm+, total alkalinity: 80ppm carbonate: 240ppm

-No Total or free chlorine (bromine) iron, copper, mercury, lead, zinc, fluoride, chlorine, manganese, QAC/QUAT, hydrogen sulfide, nitrates/nitrites, sulfates, ammonia or phosphates detected or at detectable levels with current equipment.

0.45um filtered

-pH 8.6

-Hardness: 50ppm, sodium chloride: 500ppm+, total alkalinity: 60ppm, carbonate: 120ppm

-No Total or free chlorine (bromine) iron, copper, mercury, lead, zinc, fluoride, chlorine, manganese, QAC/QUAT, hydrogen sulfide, nitrates/nitrites, sulfates or ammonia detected or at detectable levels with current equipment.

-No Glyphosate (Herbicide) detected or at detectable levels with current equipment.

Substance /Test	Sample Average (Unfiltered)	Sample Average (Unfiltered)	Sample Average (Unfiltered)
	A	B	C
Temperature	19.8°C	24.7°C	24°C
Dissolved Oxygen	3.5ppm	>15ppm	>15ppm
EC	995µS/cm	812.5µS/cm	845.3µS/cm
TDS	497.5ppm	425ppm	419.6ppm
pH	7.75	9.1	9
Water Hardness	62.5ppm	75ppm	50ppm
Total Alkalinity	70ppm	70ppm	80ppm
Carbonate	100ppm	200ppm	240ppm
Total Chlorine*	0.75ppm	0.75ppm	Not Detected
Free Chlorine	Not Detected	Not Detected	Not Detected
Sodium Chloride	500ppm+	500ppm+	500ppm+

Soil Samples	pH
S-1	7.5
S-2	7.5

From these results, we suspect the most likely cause of fish deaths to be from algae blooms, as the low dissolved oxygen (DO) levels in the morning, supersaturated levels in the afternoon, combined with the high pH are all classic presentation signs of algae bloom derived hypoxia. 

During the daytime, algae use sunlight to photosynthesize & produce high amounts of oxygen, however without the sun algae still respirate & utilize oxygen within the water, leading to the opposite effect during the night. After recent blooms, algae die off & the decomposition leads to even lower oxygen levels. DO levels should be measured early in the morning, around pre-dawn. Samples were unable to be taken until 8:30am, meaning the reading of 3.5ppm (already borderline hypoxic) at this hour means DO levels were most likely lower in the hours before. Larger fish, like the ones seen dead, often require at least 5-6ppm. This was also the sample taken near an active air pump (~300ft), meaning results in the majority of the reservoir were possibly lower. The test used to measure DO levels was a colorimetric reagent kit with a test limit of 15ppm,

meaning the exact levels are unknown but most likely very high in the evenings.

Photo courtesy of: https://drstdashbiologist.wordpress.com/2017/06/13/algal-blooms/

High pH

Ideal pH levels for aquatic life should be around 6.5-8. A pH of 9 is not acutely toxic, but is the upper limit & considered to stress most aquatic life when exposed to high pH conditions. This is in part likely due to the algae, however Monee Reservoir sits between two highways higher in elevation, as well being surrounded by lots of farmland, & has an active railroad crossing through the area. All of this would & is most likely contributing to higher pH levels from runoff. 

Salinity & EC

Sodium Chloride (Salt), Salinity, & Electrical Conductivity (EC) were all measured in the higher, upper threshold ranges. The above mentioned highways & agriculture is most likely the main cause for this as well. Levels are not acutely toxic, but high enough to begin stressing aquatic life. 

*Total Chlorine

Test strips showed positive for small levels of total chlorine but negative for free chlorine. Additional test strips were used & showed negative for free chlorine. This could be caused from chloramines or other similar compounds left over as a result from reacted previous free chlorine. Sources could be from the highways/railways, or chemical sprays (one passerby commented they'd seen staff “spray herbicides for lilipads” although this hasn't been confirmed). Only one test type was available for total chlorine, meaning this also could’ve been false readings as we were unable to confirm total chlorine with other methods, although consistent readings does raise some suspicion.

Conclusion

In total, the water does not appear to be chemically toxic. Visual observations under a microscope showed an abundance of snails & other microorganisms like worms & copepods, as well as good clarity. The presence of algae, DO patterns & pH indicate fish deaths are most likely from hypoxia from algae blooms. The high pH & high salinity is also likely contributing to aquatic life stress. Only one test for the herbicide glyphosate was available with a limited detection threshold, & tests for other contaminants were unavailable. Further testing would need to be completed to confirm these results, as well as a deeper analysis overall to get a more accurate picture. This is only a simple water screening meant to begin narrowing down possible causes.

*Disclaimer: This report was published & made publicly available with the intent of spreading awareness & education.